A combination of the Slotted-ALOHA method and the Code Division Multiple Access (CDMA) method is known as a satellite communication access method used in a wireless communication system for performing communication between a base station and a plurality of communication terminals.
According to this satellite communication method, the transmission-side communication terminal synchronizes transmission data spread using an individually allocated spreading code with slots on the time axis and transmits at predetermined slot intervals, and the receiving-side base station uses the same spreading code as that of the transmission-side to despread the received data and to extract the required data.
By combination of these two access methods, even if the plurality of communication terminals transmits packets using the same slot, if the spreading codes are different, the base station is able to extract data from the received signal. Thus decrease is possible of the incidence of the occurrence of a state in which the base station is unable to extract data from the received signal due to collision of packets transmitted from the plurality of communication terminals.
However, when the plurality of communication terminals uses the same spreading code and packets are sent using the same slot, packet collision still occurs, and the extraction of data from each packet becomes impossible.
Patent Literature 1 discloses technology by which, in order to decrease the occurrence of packet collision, a base station sends delay time information to each terminal, and each communication terminal adjusts the timing of transmission based on the delay time information acquired from the base station.